How To Make Your Batteries Last as Long as Possible

Your battery bank will likely be one if the most expensive components of your “off the grid” energy system. They are also the only piece of the system that will need replacing on a regular basis. Every extra day you can squeeze out of your battery bank will reduce the overall cost of your renewable energy system.

If you have already damaged your batteries, it is worth checking these guys out and using their advice….

Deep cycle solar batteries have lead plates (and other materials) immersed in electrolyte. The more surface area on the plates and the more electrolyte, the more capacity the battery will have. Larger deep cycle batteries will have more plates, thicker plates and more electrolyte to give the batteries more capacity.

Our job as battery owners is to preserve the surface area on the plates. If we can keep the plates clean and in good shape, we can extend the capacity and longevity of the battery.

PLEASE DON’T GET OVERWHELMED!

There are many tips below but some are a one-time deal such as setting the correct bulk voltage. Once it is done once, you will not need to think about it again.

Some are more complicated or require your attention on a regular basis. The more you work at it, the longer your batteries will last. However, if you work many hours per week for a few years to maintain your batteries, it might make more sense to buy new batteries as needed and spend the extra time with your family.

Your first battery bank will take the most abuse and last the least amount of time. As you get better at battery maintenance and monitoring your power system, you will be able to make your batteries last longer.

Here are many methods of making your batteries last longer than even the manufacturer’s specifications:

Do not have more than three parallel (or two with VRLA) strings.

When you are building a battery bank, you will often have to wire your batteries in parallel to get the required amp hours. Most experts agree that paralleling battery strings is not the best idea. Having two parallel strings will not cause any issue but three or more (especially with VRLA or sealed like gel cell or AGM batteries) will result in unbalanced batteries as some will take most of the charge and discharge, while others will sit almost dormant. If you must use three or more strings in parallel be sure to tie them together as shown in the diagram and equalize a little more frequently than usual. (Click on the picture to zoom in.)

Do not discharge your batteries any more than 50%.

In fact, the less you discharge your battery bank, the longer it will last.

Every time a battery is discharged and re-charged, it is referred to as a cycle. Every time a battery is cycled it wears. The deeper the cycle, the greater the wear. If your batteries were designed to cycle 500-800 at 80% depth 0f discharge (meaning 20% of the capacity left in the battery), they may last 2000 or more cycles at 20% discharge (80% still left in the battery). Cycling a battery actually removes a tiny amount of material from the plates as well as causes a buildup of sulfate on the remaining plate material. Both of these actions decrease battery capacity and longevity.

If you are using flooded lead acid batteries in your power system it is very important to keep them topped off with distilled water. There are two reasons. First, the plates must always be covered with electrolyte. Second, if water is not added, the electrolyte will become too concentrated leading to premature battery failure. If the battery’s plates are exposed to the air for even a day or two, they will be damaged. If this happens, fill them asap and perform an equalization charge to mix the electrolyte.

PLEASE NOTE:Only use distilled water. Using tap water will add dirt, dissolved solids, elements and minerals to your electrolyte.

Make sure the bulk/absorption voltage is correct.

ALWAYS follow the manufacturer’s instructions when setting up your solar charge controller, wind turbine controller, inverter charger or other charging equipment. The settings you need to be concerned with are the bulk/absorption voltage, float voltage and equalization voltage. The manufacturers test their batteries regularly as well as receive feedback from trusted solar installers to come up with the best voltages for your battery bank.

If you set the voltages too low, the batteries will have a build up of sulfate on their plates that will harden over time and become impossible to remove. If you set the voltages too high, the batteries can be overheated warping the plates or have more plate material removed than necessary.

Temperature has a huge impact on battery voltage readings. A cold battery will have unrealistically low voltage readings compared to a warm battery. As battery temperature increases, so does the at rest voltage. A high voltage reading from a hot battery does not mean it is full, it just means it is hot. A battery temperature sensor will make the necessary adjustments to charging voltages as the temperature changes.

Let’s imagine you have two Surrette S460s in series to form a 12 volt battery bank. Surrette tells us to bulk/absorption charge these batteries at 14.4 volts. However 14.4 volts is only correct at 25 degrees Celsius (C). As the battery temperature decreases, it takes more voltage to fill the S460s. At 0 (zero) degrees Celsius (C) it will take 15.0 volts for bulk/absorption. If your controller were to charge these cold batteries at 14.4 volts, they would always remain undercharged. You cannot make these voltage adjustments manually as the battery temperature can change many time throughout the day.

Another mistake is using a charge controller with a built in temperature sensor. Although it is better than nothing, the inside of your charge controller will not be the same temperature as your batteries. Make sure you have a temperature sensor that mounts to the batteries and install it directly against the case (below the level of the electrolyte) of one of the batteries in the middle of the battery bank.

DO NOT allow your batteries to get hot.

Although having a hot battery will increase its capacity it will destroy your batteries fast. The colder a battery is, the longer it will last. In fact a typical deep cycle battery at -30 degrees C will have a 60% longer lifespan than at 25 degrees C.

We have already mentioned that batteries are rated at 25 degrees C. Every 8 degree C rise (over 25 degrees C) will cut the battery lifespan in half. Just by having your battery bank at 33 degrees C you will only get half the rated cycles.

If you were thinking of installing your batteries in an insulated box please be cautious of overheating.

Never charge a battery that is over 35 degrees C.

As we have discussed, the colder a battery bank is, the better its lifespan will be. However the real damage is done when a battery is actually being charged when it is hot. If your batteries are over 35 degrees C do not allow them to be charged. Do whatever is necessary to cool them first.

Do not try to get away with the minimum amount of batteries.

Trying to save money in the beginning by installing the bare minimum battery bank will always end in disaster. Your system will never perform well. Your spouse will always wonder why your electricity is always turning off. Having an undersized battery bank will mean deep cycles and very limited battery life. In the short term, you will save some money but in the long term it will cost you when you have to buy a second battery bank. Almost everyone makes this mistake. You will not!

Cold batteries are fine.

You do not have to worry about your batteries getting cold. They do not mind at all. It is true that capacity will be reduced but as we discussed above, the battery bank itself will have a lengthened lifetime. Even if your batteries are located in a cold environment, they are likely staying warm enough as batteries produce a lot of heat while being charged and discharged.

Make sure your absorption time is correct.

Bulk charging will bring the battery bank to a bout 80% state of charge (SOC). The absorption charge is the time spent charging once the voltage has reached the bulk voltage, All three stage chargers allow you to program how long you want your absorption charge to be. The time setting for absorption charging is critical to battery longevity. If this is not correct, your batteries will never reach 100% state of charge causing sulfation (if setting too low) or plate wear (if setting too high). What is bulk charging versus absorption charging?

Do not charge at more than 13% of your battery’s C20 AH rating.

Charging batteries at too high of a current will overheat them and caused damage by warping the plates and removing plate material. The best way to decide how much of a current your batteries can handle is to find the battery’s C20 amp hour rating (What is a C20 rating?) and multiply it .13 (13%). This will be the maximum charge rate you should put in your batteries.

For example: You have 4 Trojan L16s in series to make a 370 AH 24 volt battery bank. You just bought a brand new shiny Magnum Energy MS4024-PAE inverter that includes a 105 amp battery charger. Using your 5000 watt cheapo gas generator, you could easily force the full 105 amps into the battery bank. But should you?

Realistically you should never force more than 48 amps into this battery bank. This includes any wind power, solar power, water power and/or generator power. All charging sources totaled should not be more than 13% of the battery bank’s C20 rating. If it happens on occasion, it will not likely cause noticeable damage as the current will automatically limit itself as the bulk voltage is reached. But chronic over current into your battery bank will cause irreparable damage.

Please note: Some battery manufacturers only allow 10% of the C20 AH rate for the bulk current setting.

Monitor specific gravity (S.G.) regularly

When purchasing your new bank of batteries you should also get a quality battery hydrometer that has built in temperature compensation. Knowing the specific gravity (S.G.) of the electrolyte in your batteries can tell you the health of your batteries and if your charging them properly. All battery manufacturer’s provide the recommended S.G. for your batteries. If your readings are high, the battery will have increased capacity but decreased lifespan. After equalizing your batteries, the S.G. readings of all of the batteries should be the same. If they are not, it could mean your equalization was not successful. It could also mean you have a bad connection or bad cell or cells in a battery.

When the batteries are full, the S.G. readings will be the highest. When they are discharged, they will be the lowest. After S.G. monitoring, you will get to know how your batteries perform compared to their specific gravity readings. Specific gravity readings that drop unusually fast (while being discharged), can be a sign your batteries are sulfated.

Do not allow corrosion on your posts, connections and cables

Just like your car battery, your solar batteries can have a buildup of corrosion on the terminals, cables and connections. Corrosion, being less conductive than copper, can make it harder for current to pass thru your battery cables and into your inverter, solar system or wind turbine etc… The other problem with corrosion is that it can build up from the positive to the negative post making a connection between the two posts.

Although the corrosion is less conductive than lead or copper, it is still conductive allowing current to pass from one terminal to another. Enough corrosion can drain a battery within hours.

Always keep an eye on your battery bank.

Every so often take a look inside the battery box and see how things are going. Is there any leakage? Does anything look unusual? Does it smell unusual or burnt? Is there excess liquid on the tops of the batteries? Any corrosion?

Feel the tops and sides of your batteries. Are there any hot spots? Does one battery feel hotter than the others?

Look at and feel the battery cables and their crimped/soldered lugs. Are they hot? Do they look like they have been hot in the past? It is very common for the lugs crimped on the end of a cable to begin to fail and get very hot. They can get so hot they melt the battery terminals off.

The last check is to remove all the battery caps and check the water levels (if you have flooded lead acid batteries). While you are in the battery, look at the plates and the electrolyte. Does it look clean? Does it look the same as all the other batteries?

It may seem like a lot of work to take good care of your batteries but it can be worth it due to the extra money you will save by not needing to purchase a battery bank until you get every kWh out of it.

When you consider that most solar equipment will last 20 years or more, taking the extra time to care for your batteries is a good use of your resources and time. It will also be fun to brag to your friends about long you made your battery bank last.

Hello,
Thanks for this article. I re-wired our 3 string 24 volt battery bank about a year ago with cables connecting each string as shown in your diagram. It is working great (more even charging), but I got a comment yesterday on my youtube video showing this wiring matrix as dangerous? Here is the quote:

“It’s quite dangerous. The extra cables should be small and use automotive fuses. Then use large hrc fuses between banks. Currently fault current can flow by several routes, with nothing to stop it. Major fire risk﻿”

Well done on your battery bank series/parallel installation. You have done everything perfectly.

The only thing I would add is a Class T 200 amp? fuse and holder as close to the the positive bus bar as possible inline on the cable going to your E Panel. I am assuming you are using 2/0 battery cable. It is hard to tell in the video. If the cable size is different please let me know and I will recommend a different size fuse.

Ignore the comment and the video. Using automotive fuses in a residential system is against code everythere and reducing the size of your parallel connections is counterintuitive.

Great job on the video and your system. Solar would not be where it is today without the greats Robin, Mary and boB and the rest of the staff at Midnite Solar.

Keep up the good work and seriously consider adding the Class T fuse if you have not already done so.

Your battery bank should be balanced well and provided years of service assuming you keep them charged and don’t abuse them.

Thank you for writing. For your battery bank you could make 7 groups of 4 batteries. Each group of 4 batteries wired in series will provide 48 volts. Then tie all the positives of each group together and then tie the negatives of each group together.

For the solar modules I would need to know what brand and type of charge controllers you are using. I would highly recommend using MPPT charge controllers and installing 3 solar panels in series and having 10 groups of these. Please let me know what charge controllers you have in mind and I will try to help you further…Jody

Hello Jody,
I am so glad to have found this site. Your information is most helpful. I am not at my system site right now but when I get home I have a flurry of questions I’m going to ask. My system is currently killing my batteries and I’m hoping you can help me figure out why! Will give you all my system info soon. Thank you so much for this resource. GHI

Keeping the battery full is easy if you don’t take anything out of it. If the battery is just sitting there it might lose 1 to 5% of its AHs or 27-138 watt hours per month, depending on your battery’s chemical make up, condition and age.
However I am guessing you will be using that battery to operate loads such as lights, radio communications, computers, pumps etc. that are typical in an off the grid power system.

The short answer is “You need enough solar modules to replace the energy you have removed from the battery plus a few percent for inefficiencies in the system and a few per cent for natural and normal losses in the battery”.

The best advice I can give you is figure out how much power your system will need to provide in watt hours or kilowatt hours and install enough solar modules to make twice that amount of power while also considering:

How many hours of sun do I get per day?
Are my solar modules installed at the right vertical angle and horizontal angle to the sun?
Is this a summer application, winter application, or both?
Is there anything in the way to shade the solar modules?

As you can see, things can get complicated quickly. Hope I have at least helped a little. Good Luck…Jody

Hi,
The 10-13% of total battery bank AHs is from manufacturer’s specifications for flooded (lead acid) batteries. All manufacturer’s recommend the maximum charge you should use to charge their specific battery and these maximum’s usually fall between 10 and 13 percent. If your particular battery’s owner’s manual (or manufacturer) recommends something else, then follow that. If not then use a bulk charge of somewhere around 10-13% maximum. Less will not harm the battery. More could possibly overheat the battery and cause damage to the lead plates such as warping them or removing too much plate material. Jody

Thank you for your comment. Keeping your batteries cool in a hot climate is not easy. If you decide to use a standard battery box make sure the batteries have good spacing around them and ventilation. Don’t charge them at any higher rate than 10% of their C20 amp hour rating. One cooling idea is to bury 20 or more feet of 3 or 4 inch sewer pipe as deep as you can with one end of the pipe vented into your battery box and the other end coming out of the ground in the shade. Use a small solar powered fan to push air through the sewer pipe that is buried where it will be cooled and the cool air will go into your battery box. You could even get more high tech by circulating water through a loop underground and have your batteries in a bath of the water that is cooled by the underground piping. Would need a low wattage dc circulating pump and a small solar panel. The air method is much easier.

I just learned from one of our fellow off gridder’s Ricardo who lives in Portugal how he keeps his batteries cool. He deals with 40 degree C temperatures. He put his battery bank in a chest freezer (deep freeze). He added an extra thermostat that turns the freezer on when the temperature goes above 20 degrees C. Now his batteries are never above 20 degrees C and the freezer only operates about ten minutes per day. He is now in the process of adding water to the bottom of the freezer up to 2/3 the height of the batteries to see if it works better. As the batteries are now in a sealed container (the freezer) there is the danger of hydrogen gas building up. This can easily be corrected by adding a 12 volt vent fan that is controlled by a relay driver built into your charge controller or inverter.

Hot batteries will destroy themselves fast. Even high end batteries will not last more than a few years at 30-40 degrees Celsius.

Please let me know how you decide to keep your batteries cool and how effective it is. Take care… Jody

what will be the minimum charge rate for a battery bank, for example I own a 875 Ah C5 battery bank and my panels add up to 25 amps peak , assuming that I can cover what I consume daily is there a minimum amount that has to go in to batteries?

Hi Alexandros,
Thank you for your comment. There really is no minimum that must go into your batteries as far as amperage is concerned. You need to be able to replace what you take out plus some electricity that is lost to inefficiency (heat). A fairly new set of lead acid batteries will lose about 4-8% of the energy your solar modules create. For every kilowatt hour you create about 920-960 watt hours will actually go into your bank. As long as your array is able to bring the battery voltage up to the bulk voltage (as per your battery manufacturer) you will be fine. If you notice that your battery voltage never gets that high, it is time to buy more modules. Hope this helps…Jody

I read this article with great learning and agree with everything spoken especially with excess heat. I live in Algarve Portugal my first 24V battery were Trojan L16H-AC x 4 they only last 2 years as Morningstar MPPT controller report battery 40 deg C heat in the summer when charging afternoon. Now for 2nd set of same battery I put in chest freezer with separate temperature control set to 20 deg C and below this relay cut power to freezer. Batteries now never go above 30 deg C in day and cool to 20 deg C at night freezer lid is open little to let out some heat and gassing and is on maybe 10 mins per hour in the day so use little energy. I think maybe to try filling freezer with water up to 3/4 height of batteries if no leaks then this maintain lower temperature better I try this next.

Ricardo,
You are a genius! That is the smartest idea I have seen yet to keep your batteries cool. Heat destroys batteries so fast. I am sure there will be a few folks who will worry about hydrogen gas building up and causing danger to you but you could easily add a small 12 volt vent fan and run it off the relay driver in your charge controller to only operate when the voltage is high enough to produce gas. The water should act as an excellent heat sink and make your cooling system work even better. Let me know if you have tried it yet. Sorry for the delay in replying to you. Your comment was lost in hundreds of spam comments. Someday I will learn how to get rid of all the spam comments but until then I will go through them one at a time. Take care…Jody

P.S. Please get in touch with me as I would love to hear how your system is working in Portugal.

Nice for the reply I can tell you since writing the initial post above I fill freezer with water to level with top of battery plates this works much better than air my bank does not go above 30 C now even when 45 C in the afternoon and cools nicely to 20 at night. Yes, it uses more energy than before but this is experimentasl after disaster with first set of batteries lasting only 2 years and besides, when it is ambient 45 C outside the sun is full on and producing 60A through controller ( avrge 20A to house and 40A charge to batteries). I may try to reduce freezer water temp to 15 or even 10 C but this may mean freezer on 24/7 so even more energy used 100W when on. If you want me to take pics let me know hope this helps others

Awesome idea but you are right, the supermarket freezers are super expensive. At first I thought you were keeping the lid closed which worried me about hydrogen gas build up. But opening the top or removing the top is a great idea. Or you could cut a hole in the side of the freezer for an air inlet (if you know where the cooling lines/condenser are). Then cut a hole in the top (to vent the gas) and add a 12 volt fan made by Zephyr Vent Fans (http://zephyrvent.com/) that is operated by a relay driver in your inverter or voltage controlled switch like the Morningstar relay driver. When the voltage gets high enough that the batteries will gas, the fan will turn on automatically to vent the hydrogen. You could also make your own vent fan with a 12 volt muffin fan (computer cooling fan) and a piece of ABS/PVC or other plastic pipe. Thanks so much for sharing your great ideas…Jody

That is great. The cooler your batteries are the longer they will last as you already know. I am glad the water helped. That was another great idea. 45 C is really hot. I go to Haiti regularly and have the same problem with battery banks being destroyed by the heat. If you have the extra energy it makes sense to try and cool the batteries to 15 or even 10 C. The lower the temperature the better. The lower the temperature of your battery the lower the capacity in amp hours but the longer the batteries will last. In Canada our battery bank can go down to -20 C but it does not hurt the batteries at all. They just do not have as much storage capacity. At -20 C the battery bank would probably last at least twice as long as it would at 20 C. Batteries love cold.

I would love for you to email me some pics to jody@solarhomestead.com. I would love to write an article about the different ways to keep your battery bank cool and I will give you all the credit for your photos and your idea. You are a very smart man…Jody

new bank = 8 250ah agm at c100 it dos not give any other rating, 2 sets of 4 giving me 48v, q 1 would it do any harm to link up the pos negs to help even charge and discharge ? so i have an 500ah bank, i make the max charge 65amps, i can never do that my inverter charger is max 35amps but dont use it normaly in summer, i have solar wich can give me 26+amps going to a zantrax c40 with a bts fitted, my discharge is very lite, inverter about 1,8a fridge freezer 2,2a water pump 600w and washing machine of wich i hot fill, pc and tv and lighting is all led or low consumo, and i go dead at bedtime all at rest at the moment checking bat volts after around 6 hours rest 50,8 around 20 to 25 % the bats are 62kls each thats almost half a ton, uff i have made all the mistakes and more, thanks for all the info very helpfull peter

Thanks for your comment. I am assuming you are using 12 volt 250 AH AGM batteries. You have them wired in 2 sets of four batteries each making a 48 volt battery bank at 500 amp hours. So far you are doing everything correct. You are correct that you would not want to charge at a higher rate than 65 amps (13% of your battery bank’s AH rating) and you are charging at 26 amps max with your solar array and you have a 35 amp inverter charger for the winter months.

Tying (linking) the positives and negatives together as in the diagram above is a good idea. It is not super important as you only have two parallel strings but it will still help balance the load on all your batteries.

It sounds to me like your system is working perfectly. The at rest voltage of 50.8 volts after the end of the day and a 6 hour rest is great. If the temperature is cold the at rest voltage will be lower. If the batteries are hot the at rest voltage will be higher.

One last comment is about the age of your batteries. If they are brand new you can expect them to get better for the first six months or so. New batteries take time to come up to their full potential.

Hope you are enjoying your solar system and hope to hear from you again soon…Jody

thank you jody for your reply, about my 8 x 12v agm 250 ah bats, they are a new set, about 2 moths old now, yes about batery temps, afraid that was one of my mistakes in the past, im in spain mallorca and it can get hot in summer 40c+, my soler regulatter is a xantrax c40 and i read about the bts, batery temp sencser, all the so called experts in spain only 1 had a bts but did not send to mallorca, i had replies saying it was not important, but i found an american company that sent one to me, and fitted it and was amazed how quik it started to work, and cut the charg volts amps down a little, i also fitted a little led temp that works of its own batts, and that at times reading 31c+ and its on the same batt as the bts, my inverter is a 4k,w, big and 38 kls, of wich i bought direct from mustups china and works really good , cost around 500+$ diliverd by dhl and they did the customs for me, as its much cheeper, so i put a small hole in the outside wall for it to suck fresh air in, also i have installed the batts on end because of space, but all seems good at moment, its the best set up i have had in 19 years, and a big yes i made lots of mistakes, its good to read what every one is doing one thing i can advise is for people buying a genset is if you dont have any 3 fase never buy a three fase geny, you just can not balence them up right and they will run bad and blow 1 fase, most good makes of alternaters you can change the brigdes to mono fase, ok hasta lluego peter

Glad things are working out for you. There is so much to learn when you first decide to build a solar system. And there is a lot of bad information on the internet. Have you been reading how Ricardo from Portugal is keeping his batteries cool? It is very important to keep your batteries as cool as possible. Heat will destroy your new battery bank very quickly.

That is great advice about the generators. I hear it all the time that you can buy a three phase generator and just use one of the phases. I used to build water turbines and sometimes we would use only one phase of the three phase generator for a turbine. Because we were only using a small portion of the alternator’s output it worked fine but if you are using much power the alternator will always be unbalanced and eventually be destroyed. Thank you for that great advice.

If you only have a three phase unit the best option is to buy a three phase transformer that will reduce the alternator’s voltage down to your equalization battery voltage and add a three phase bridge rectifier to convert the AC to DC. Now you will have DC electricity to directly charge your batteries and your alternator is balanced. It is similar to how an automobile alternator works.

Then you MUST add a dump load or have a means of shutting down the generator when the batteries are full.

Thank you again for your great advice and keep up the great work. If you ever want to communicate my direct email is jody@solarhomestead.com.
Take Care,
Jody

I’m a retired electrician. This a completely new horizon for me and I find it extremely interesting. I live in a densely wooded area and do experience grid outages at times. I plan on using my system to power one circuit in my home and one circuit in a out building. I found this article extremely interesting and very helpful.